A typical vane-type camshaft phaser for changing the phase relationship between a crankshaft and a camshaft of an internal combustion engine generally comprises a plurality of outwardly-extending vanes on a rotor interspersed with a plurality of inwardly-extending lobes on a stator, forming alternating advance and retard chambers between the vanes and lobes. Engine oil is selectively supplied to one of the advance and retard chambers and vacated from the other of the advance chambers and retard chambers by a phasing oil control valve in order to rotate the rotor within the stator and thereby change the phase relationship between the camshaft and the crankshaft. One such camshaft phaser is described in U.S. Pat. No. 8,534,246 to Lichti et al., the disclosure of which is incorporated herein by reference in its entirety and hereinafter referred to as Lichti et al. As is typical for phasing oil control valves, the phasing oil control valve of Lichti et al. operates on the principle of direction control, i.e. the position of the oil control valve determines the direction of rotation of the rotor relative to the stator. More specifically, when a desired phase relationship between the camshaft and the crankshaft is determined, the desired phase relationship is compared to the actual phase relationship as determined from the outputs of a camshaft position sensor and a crankshaft position sensor. If the actual phase relationship, does not match the desired phase relationship, the oil control valve is actuated to either 1) an advance position to supply oil to the retard chambers and vent oil from the advance chambers or 2) a retard position to supply oil to the advance chambers and vent oil from the retard chambers until the actual phase relationship matches the desired phase relationship. When the actual phase relationship matches the desired phase relationship, the oil control valve is positioned to hydraulically lock the rotor relative to the stator. However, leakage from the advance chambers and the retard chambers or leakage from the oil control valve may cause the phase relationship to drift over time. When the drift in phase relationship is detected by comparing the actual phase relationship to the desired phase relationship, the oil control valve must again be actuated to either the advance position or the retard position in order to correct for the drift, then the oil control valve is again positioned to hydraulically lock the rotor relative to the stator after the correction has been made. Consequently, the position of the rotor relative to the stator is not self-correcting and relies upon actuation of the phasing oil control valve to correct for the drift.
U.S. Pat. No. 5,507,254 to Melchior, hereinafter referred to as Melchior, teaches a camshaft phaser with a phasing oil control valve which allows for self-correction of the rotor relative to the stator as may be necessary due to leakage from the advance chamber or from the retard chamber. Melchior also teaches that the valve spool defines a first recess and a second recess separated by a rib such that one of the recesses acts to supply oil to the advance chamber when a retard in timing of the camshaft is desired while the other recess acts to supply oil to the retard chamber when an advance in the timing of the camshaft is desired. The recess that does not act to supply oil when a change in phase is desired does not act as a flow path. Melchior also teaches that the valve spool may be moved axially or rotationally in order to direct oil to the required chamber. In order to rotate the valve spool, an arm is provided which is sensitive to engine speed. The arm is geared to the valve spool, and consequently adjusts the rotational position of the valve spool as directed by the position of the arm based on engine speed. However, rotating the oil control valve directly and mechanically by an arm that is sensitive to engine speed may not be adequate for operation because modern internal combustion engines rely on many parameters, typically provided by various sensors which monitor various aspects of engine performance, processed by an electronic processor, for example an engine control module, to determine a desired camshaft phase. Consequently, it is desirable to have an actuator which is able to rotationally position the phasing oil control valve while taking into account any number of engine performance indicators.
U.S. Pat. No. 4,770,060 to Elrod et al., herein after referred to as Elrod et al., teaches a camshaft phaser which uses a gear drive mechanism, and more specifically a harmonic gear drive mechanism, actuated by an electric motor, to alter the phase relationship. The electric motor rotates together with the camshaft, and consequently includes slip rings in order to transmit electricity to the electric motor. When no change in phase is desired, no electricity is transmitted to the electric motor, thereby not inducing movement in the gear drive mechanism. However, it may not be desirable to use slip rings which may encounter issues in implementation.
U.S. Pat. No. 6,981,478 to Shafer et al., hereinafter referred to as Shafer et al., also teaches a camshaft phaser which uses a gear drive mechanism actuated by an electric motor to alter the phaser relationship. However, unlike Elrod et al. which requires slip rings to power the electric motor, the electric motor of Shafer et al. does not rotate with the camshaft, thereby eliminating the need for slip rings. However, in order for the electric motor of Shafer et al. to remain stationary, an output shaft of the electric motor which engages the gear drive mechanism must be speed matched with the input side of the camshaft phaser when no change in phase is desired, thereby requiring the electric motor to be continuously operated. Conversely, when a change in phase is desired, the speed of the electric motor is increased or decreased as needed to induce the appropriate motion on the gear drive mechanism which will result in achieving the desired change in phase relationship. However, it may be desirable to only operate the electric motor, without the need for slip rings, when a change in phase is desired.
What is needed is a camshaft phaser which minimizes or eliminates one or more of the shortcomings as set forth above. What is also needed is an actuator which minimizes or eliminates one or more of the shortcomings as set forth above.